1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the same and to a magnetoresistive device, a thin-film magnetic head, a head gimbal assembly, a head arm assembly and a magnetic disk drive each of which incorporates the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Recently, developments in read heads using TMR elements have been sought to conform to further improvements in areal recording density.
Typically, a spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to a plane of each layer making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. On the other hand, developments have been made for another type of GMR heads each having a structure in which a sense current is fed in the direction intersecting a plane of each layer making up the GMR element, such as the direction perpendicular to a plane of each layer making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element. A read head incorporating the above-mentioned TMR element has the CPP structure, too.
Compared with a TMR element, a typical CPP-GMR element has a very low electrical resistance since all the layers making up the CPP-GMR element are made of metal materials. Therefore, the typical CPP-GMR element has a problem that the magnetoresistance change amount is small, wherein the magnetoresistance change amount is the product of the electrical resistance and the magnetoresistance change ratio (MR ratio). Furthermore, in a read head using such a CPP-GMR element, the output voltage proportional to the magnetoresistance change amount is low, which is a practical problem of the CPP-GMR element. To solve this problem, a variety of proposals have been made for providing a multilayer pinned layer and/or a multilayer free layer.
For example, Japanese Published Patent Application 2002-92826 discloses a technique wherein at least one of the pinned layer and the free layer of a CPP-GMR element is formed as a layered structure made up of ferromagnetic layers and nonmagnetic layers alternately stacked. This technique provides a great number of interfaces between the ferromagnetic layers and the nonmagnetic layers, and the effect of scattering of electrons depending on spins at the interfaces increases the magnetoresistance change amount.
Japanese Published Patent Application 2004-6589 discloses a technique wherein a very thin layer containing oxide, nitride, oxinitride, phosphide or fluoride is inserted inside a ferromagnetric layer (the pinned layer and the free layer) of a CPP-GMR element or inserted to the interface between the ferromagnetic layer and a nonmagnetic conductive layer of the CPP-GMR element. The thin layer modulates the band structure of the ferromagnetic layer near the thin layer, and causes the spin filter action of electrons. The objective of this technique is to achieve a CPP-GMR element having a great magnetoresistance change ratio without increasing the element resistance.
Japanese Published Patent Application 2002-359412 discloses a technique wherein a high resistance layer is provided across the passage of a sense current in a CPP-GMR element. The objective of this technique is to increase the magnetoresistance change amount by increasing the element resistance.
It is possible to increase the electrical resistance or the magnetoresistance change ratio of the CPP-GMR element by providing a multilayer pinned layer and/or a multilayer free layer. However, the magnetoresistance change amount of conventional CPP-GMR elements obtained so far is not large enough.
Furthermore, it is also required that the CPP-GMR element have a high magnetic field sensitivity (the magnetoresistance change/the external field change). For achieving a high magnetic field sensitivity of the CPP-GMR element, it is required that the free layer have excellent soft magnetic characteristics. To be specific, excellent soft magnetic characteristics mean that the magnetostriction constant and the coercivity are small.